Lamellar orientation and interlamellar cracks in co-crystallized poly(ethylene oxide)/poly(L-lactic acid) blend

Abstract

The correlation between the lamellar orientation in ring-banded spherulites and crack patterns in blends of two crystalline polymers crystallized in two steps was investigated. The two polymers were poly(ethylene oxide) (PEO) and poly(L-lactic acid) (PLLA) with a low molecular weight. The techniques used for this investigation included polarizing light optical microscopy, scanning electron microscopy and atomic force microscopy. Crack formation was influenced by the ring band patterns in PLLA and overlapping or impingement between PEO spherulites upon cooling. By extracting the water-soluble PEO from the PEO/PLLA blend, the inner morphology of the lamellar textures was further revealed to have a complex pattern. It consisted of the outer macro-lamellae with a flat-on width of 20 lm and twisting micro-lamellae with a 1-2 lm edge-on width. As the excess PEO was etched off from the valley and the interlamellar crevices of the PLLA lamellae, the outer macro-lamellae plates (B20 lm flat-on width) exhibited up and down periodicity with flat-on orientation. However, the micro-lamellae (with 1-2 lm edge-on width) with twisting were visible inside the cracks. Similar analysis of a PEO-rich composition, PEO/PLLA (80/20) blend with irregular ring bands and no cracks, revealed that the outer macro-lamellae were not present. However, all micro-lamellae twists in the ridge regions irregularly protrude out to create zig-zag banding.

Lamellar assembly and orientation-induced internal micro-voids by cross-sectional dissection of poly(ethylene oxide)/poly(L-lactic acid) blend

“…The OM graphs for same samples are shown at right side of the POM graphs for comparison of contrast between POM and OM imaging interpretation on the etching effect. For Figure 2a and Figure 2b, the water-etching affected not only on the bright bands but also dark band; however, the etching took place more severely and circumferentially on the entire dark bands, which was indicated from our previous study [16]. By comparison, the bright band (ridge band) remained relatively unaffected by water etching but the bright band became coarser after etching.…”

“…As PEO and PLLA co-crystallized in two stage-crystallization, the role of PEO in the crystallized PEO/PLLA blend can be regarded as a temporary moldable filler among the PLLA lamellae. In continuing the previous study [16], correlations between the rings, cracks and lamellar pattern in ring-banded spherulites in PEO/PLLA blends were probed in greater details by comparing the lamellae and crack behavior of PLLA crystallizing with different PEO contents in blends. This study mainly focused on the PEO-rich blend compositions, and within this composition range, the blends were known to show unusual radial and irregular crack patterns instead of concentric cracks [16].…”

“…In continuing the previous study [16], correlations between the rings, cracks and lamellar pattern in ring-banded spherulites in PEO/PLLA blends were probed in greater details by comparing the lamellae and crack behavior of PLLA crystallizing with different PEO contents in blends. This study mainly focused on the PEO-rich blend compositions, and within this composition range, the blends were known to show unusual radial and irregular crack patterns instead of concentric cracks [16]. Objectives were further probing the mechanisms of lamellar patterns and orientation in ring bands responsible for inducing special types of surface rings and interior lamellar assembly and cracks in crystallized and cooled polymers at micro-scale, by delicately removing PEO from crystallized PEO/ PLLA blends, but without altering the already fixed PLLA crystals, to expose more details of interior morphology.…”

The relationship between morphology and impact toughness of poly(l-lactic acid)/poly(ethylene oxide) blends

“…The mechanical properties of PLLA/PEO blends determine their broader applications. 32,33,42,43 It is reasonable to expect that the Izod impact toughness of PLLA is enhanced by the blending. In addition, the mechanical properties of PLLA have been This is the Pre-Published Version demonstrated to be significantly affected by its morphology, which is strongly controlled by the processing parameters.…”

“…The morphology of their blends under different annealing conditions has already been studied in recent years. [29][30][31] Water-etched annealed PLLA/PEO blend thin films were examined to determine the PEO distribution within the PLLA spherulites with more than 50 wt% of PEO. 31 The banded structures in the spherulites of the PLLA/PEO blends were also revealed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and a profilometer.…”